Method of operating a blast furnace



July 6, 1965 K. w. STOOKEY METHOD OF OPERATING A BLAST FURNACE 3 Sheets-Sheet 1 Filed Feb. 11, 1963 y 1965 K. w. STOOKEY 3,193,379

METHOD OF OPERATING A BLAST FURNACE Filed. Feb. 11, 1963 3 Sheets-Sheet 2 INVENTOR. KENNETH W. STOOKEY MTFOQNE?" United States Patent Office 3,l93,379 Patented July 6, 1965 3,1%,37 METHOD OF OPERATING A BLAST FURNACE Kenneth W. Stookey, Markle, Pa. Filed Feb. 11, 1963, Ser. No. 257,580 Claims. {C 75-41) This invention relates to a method of operating a blast furnace, and more particularly to a method of increasing an equivalent to the straight line temperatures in hot blast which is utilized in operation of the blast furnace. The straight line temperature of the hot-blast is that selected temperature at which mill personnel endeavor to maintain the hot blast to achieve uniform operation of the furnace and to control the quality of the iron produced. Maintenance of the straight line temperature or its equivalent during furnace operation is highly important for regulation of hearth temperature and manufacture of high grade iron.

Conventionally, blast furnaces have a set of stoves, usually three or four, to preheat the air and thereby produce the hot blast which is delivered from the stoves to a bustle pipe through a hot blast line. From the bustle pipe, the hot blast travels through a goosenecl: to tuyeres which introduce it into the furnace. One stove at a time delivers heated air into the hot blast line until the temperature of the air therefrom is closed off and heated air from another stove which has been operating to heat up its cheokerwork is directed into the hot blast line. Accordingly, each stove of the set is similarly used with only one stove at a time providing the hot blast while the others are being heated up.

Heretofore, selection of the straight line temperature resulted from ascertaining the maximum temperature of the hot blast produced from the set of stoves and then choosing a temperature at some point below the maximum one for the straight line temperature. The point was as much as 300 to 400 F. below the maximum temperature so that achievement of the straight line temperature required delivery of by-pass air at ambient temperature into the blast line as a coolant to bring the temperature of the hot blast down to the straight line one. As the temperature of heated air from a stove dropped from cooling of its checkerwork by flow of air to be heated therethrough, the coolant air was gradually out back to compensate for this drop and to maintain the straight line temperature. When the heating ability of the stove reached a control point at which it could not sustain the straight line temperature, it was taken off and a hot stove connected into the hot blast line.

In the blast furnace, production of iron from iron ore results from an indirect reduction and a direct reduction of the ore with a proper balance between the amount of indirect and direct reduction required for maximum elliciency in furnace operation. This balance is about 55% indirect reduction and 45% direct reduction with the indirect reduction taking place above 1700 F. and below 2000" F. accompanied by release of a small amount of heat. Direct reduction occurs above 2000 F. and in a lower portion of the furnace and is accompanied by absorption of large quantities of heat. Consequently, production and maintenance of high temperatures in the direct reduction zone of the furnace are required and account for a demand for high hot blast temperatures.

It is known that higher hot blast temperatures increase the efficiency of the furnace, reduce consumption of coke per ton of metal, raise the production rate of the furnace, and also reduce the cost of the metal. However, increased hot blast temperatures for lowering coke rates encounter difficulties in that they bring about an unbalance bet-ween the ratio of direct to indirect reduction. The higher temperatures of-air blast consume less cokeand therefore generate less CO gas and other reductants which in turn is the exothermic-reaction producing gas at the top of the furnace. Since the higher temperatures at the bottom generate less CO (there can be no CO and the CO is the exothermic-producing reaction with Fe O there is a lower temperature at the top of the blast furnace with too high temperature at the bottom. The higher hot blast temperatures make available a greater amount of heat above 2000 F. but no change in the amount of heat available for the stack (between 400 and 2000'F.) or in the amount of reducing gas produced by combustion. Thus, the lower amount of coke to take advantage of the larger amount of heat available from the higher hot blast temperatures makes less heat available to preheat the burden in the stack and less carbon monoxide to carry out the proper reaction in the zone of indirect reduction.

Although more direct reduction occurs, the decrease in the amount of carbon monoxide in the hearth zone slows down the rate of the reaction there with the result that large amounts of iron ore melt before being reduced to metallic iron. This melted iron oxide comes into direct contact with the coke and is reduced to iron. Since the melting temperature of iron is higher than that of iron oxide, the iron so formed solidifies and freezes the coke particles together, thereby causing the furnace to hang. Thus, realization of benefiits from higher hot blast temperatures requires control of the flame temperature in the furnace and maintenance of a proper ratio between the heat available above 2000 F. and the heat and reducing gases available for the stack of the furnace.

Addition of oxygen enriched air to the hot blast encounters the same difficulties described above .and,'in some instances, an acceleration of same.

To overcome the hanging condition in the furnace, moisture is introduced into the hot blast or the moisture content of the hot blast is increased. This moisture in the hot blast effects an endothermic chemical reaction between water vapor and the coke which lowers the flame temperature and decreases the heat available above 2000 F. Accordingly, additional carbon must be burned in the furnace or the hot blast temperature increased to provide the heat absorbed by the chemical reaction and to avoid a loss in production rate.

The water vapor increases the amount of reducing gas, hydrogen and carbon monoxide, formed and thus brings about an increase in the amount of indirect reduction and a decrease in the amount of direct reduction.

A recent development in blast furnace operation to increase furnace output and lower costs includes injection of a hydrocarbon fuel, such as natural gas, coke oven gas, oil, powdered coal, etc., through the tuyeres for combustion in the combustion zone of the furnace. However, the amount of hydrocarbon fuel which can be effectively utilized is limited by the temperature of the hot blast. Additionally fuel combustion in the combustion zone of the furnace accompanied by a. too high temperature of the hot blast produces a hot spot or zone of fused carbon and iron around the tuyere openings. This zone tends to seal off proper reduction of the ore from the balance of the burden and prevents spreading of iron reduction reactions throughout the burden. Therefore, injection of a hydrocarbon fuel into the furnace is somewhat similar to increasing moisture content of the hot blast in that in both 3 tion systems, use of better and more expensive refractories in the stoves, installation of an additionai stove, etc. All of these have required expenditures of considerable sums of money. V 7

My invention provides realization of the benefits from higher hot blast temperatures through ability to hold the flame temperature in the furnace within a reasonable range to prevent formation of the hot spotsand by supanced ratio between direct and indirect reduction of the ore nates, cooling the hot blast by means, such as the conventional bypass air line to produce and maintain a straight line temperature effect while reducing materially expenditures for apparatus and equipment to furnish the hot blast at higher temperatures. Specifically, it is a method of operating a blast furnace which has stoves for heating air to produce the hot blast and a line for conveying the hot blast from the stoves to tuyeres for delivery into the furnace. V

'In another embodiment of my invention, I utilize as a method for controlling the combustion of the hot air blast, a combination of direct fuel injection through the tuyeres of blast furnace and also diverting a portion of such fuel for combustion within the hot air blast line between the stoves and bustle pipe. It would be preferred to burn all of the fuel with the incoming air in order to obtain an intimate mixture of the combustion product and air butthis is not always possible because of the structural limitations of' the ceramic materials. Whether the fuel is injected through'the tuyeres or burned in the combustion chamber, the thermodynamic effect on the furnace is identical. From the foregoing it will be seen that the invention obtains a complete elimination of the bypass, and I'further obtain a substitution of fuel for steam effect, as will be seen clearly from the following detailed description of the invention.

The advantage and reason for splitting the injected fuels is to overcome the laminar or Stratification effect a on furnace combustion and reactions, and its consequent adverse effect on furnace operation. Since the fuel cannot be premixed in the tuyeres, the fuel must be injected at the'tuyere noses which precludes any mixing with the air before the separate streams enter the furnace. r

In a still further embodiment of the invention, I have found it possible to introduce a quantity of steam, carbon dioxide or combination of these two materials into the flow of air before it is preheated by the stove and is therefore thoroughly mixed with the air and is uni formly heated to the same temperature'as the air. The hot blast is then adjusted both compositionally and in temperature and the quantity of steam and carbon dioxide is adjusted relatively to the temperature of the hot air blast and this combination is then injected into the hearth zone of the blast furnace which may also receive an adjustable flow of fuel as previously described.

It is possible by means of the present invention to completely eliminate the use of the heretofore essential cold air bypass and allow the full temperature from the stove to enter the furnace. For the control of the internal reactions in the furnace I add injected fuel in such relationship to the increase of temperature that I maintain an optimum thermo-chemical balance. My objective is to substitute the use of fuel for that of steam and more fuel to control the extra sensible heat in the blast air, thereby being able to use more total fuel than is presently possible. 7

Therefore, the hot air blast can be supplemented by an injection of steam and carbon dioxide, i.e., combustion products, into the air before it passes through the preheating stove, or the hot air blast can be adjusted compositionally by a fuel injection subsequently to being Additionall my invention avoids, if not elimi 7 ply of sufiicient reducing gas to maintain a proper balpreheated by the stove; or a combination of these two operations can be used.

Other objects and features of the invention will become apparent from a consideration of the following description which proceeds with reference to the accompanying drawings wherein: v

FIGURE 1 is a diagrammatical view of the apparatus for effecting adjustment of both the temperature and composition of the air blast and embodying the control features of the present invention;

FIGURES 2 and 3 are front and side elevation views respectively of one of the tuyeres which is shown in en- 7 larged detail view and illustrating schematically the flows of air and fuel respectively;

FIGURE 4 is an enlarged detail view of the lower portion of the blast furnace, bustle pipe and tuyeres for the blast furnace and illustrating the direct-reduction zone of the blast furnace, which is illustrated in cross sectional view; and,

FIGURE 5 is a section view taken on line 55 of FIGURE 4.

Referring now to the drawings, the blast furnace designated generally by the reference numeral 10 receives a substantially constant flow of air from a hot blast main 12 into which air is fed under pressure from a compressor 14 which moves the air from atmosphere through an inlet main 16 to a stove 18, there'being generally a series of stoves, which are in the process of being heated and at least one of which receives the flow of air to efiect heating of the air. The stoves are filled with ceramic checkerwork which is heated and each stove is of substantial thermal capacity so that air can be passed through the stove and be heated thereby for a period of about one hour and the system is then. operated by suitable valves so that a different stove is then used which has been heated and is in a ready condition. Thus, one of the series of stoves is in heated condition for operation at all times and the stoves are located in parallel and are isolated one from the other. 'Generally, the checker- Work of the stoves, which constitutes'the heat storage medium, is raised to operational temperature by an external blast furnace gas combustion system 20 having a line 22 which connects with the stove 13 or a stove internal combustion system, and after heating the checkerwor. the waste gas is withdrawn through line 24 and vented to'atmospherethrough a stack 2s. The air after passing through stove 13 discharges into the main I2 and its fact, the present invention is unique in that it completely eliminates such wasteful and undesirable previous practice of employing a bypass line in which the heated air from the stove is reduced in value. Nevertheless, the discharged 'air from the stove 18 is variable in temperature and it is necessary for optimum operation of the blast furnace to maintain'a substantially constant temperature of blast airfor proper blast furnace operation. The means which I employ for that purpose is to pro vide a flow of fuel, either solid fuel such as powdered coal,

liquid fuel such as petroleum,'or a gaseous hydrocarbon fuel, any one of which is suitable for injection through the tuyeres 28, through line 30 and effects an endothermic reaction within the hearth zone of the blast furnace. In this way, none of the heat of the air blast is lost by needless'dilution but instead all of the heat abstracted by the air blast from the furnace 18 is embodied in the blast furnace and none of it is wasted.

The hot blast line terminates in tuyeres 28, generally about eight in number, which receive the flow of hot blast furnace air and also receive a flow of fuel (coal dust, liquid fuel or gas) from line 30 and the two flows are injected into the hearth zone 32 which is the zone of direct reduction and must be adjusted both from a temperature and composition standpoint to provide equilibrium between the zones of direct and indirect reaction. Instead of adjusting the variable temperature hot air blast to a constant temperature, I effect the maintenance of a hot air blast reference temperature equivalent by sensing the temperature of the air blast and regulating the amount of injected fuel in accordance with the sensed temperature. Thus, when the temperature of the air blast is at its highest, the quantity of fuel is increased and the endothermic reaction is increased by correspondingly increasing the flow of fuel. Conversely as the temperature falls, the amount of fuel is decreased to reduce the endothermic reaction.

Unlike previous controls in the prior art, there is used in the present invention a thermo-chernical equilibrium condition to effect temperature control rather than diluting and thereby losing such heat by means of the conventional bypass line. As described, the injectants are highly endothermic and therefore absorb heat from the hearth to cool it. Such heat is not lost, but is effective in reducing the iron oxide in the zone of indirect reduction.

The endothermic reaction described is in accordance with the following equations:

The carbon dioxide and Water which are expressed in these foregoing reactions are obtained at least in part by oxidation products which are obtained from burning fuel within chamber 34, connecting with the hot blast main 12, said chamber 34 receiving a branch flow of fuel from branch line 36, the two branch lines 39, 36 being joined with a main fuel line 38. The two branch lines 30, 36 include valves 40, 42 which can determine the proportion of fuel divided between combustion within chamber 34 and passage through branch line 30 for combination with the hot air blast at the tuyeres 28. The total quantity of fuel which is divided between chamber 34 and in branch line 36, 36 is determined by the amount of temperature within the hot air blast. This being accomplished in the manner next to be described. The total quantity of fuel is regulated by an inlet valve 44 which is controlled by a solenoid 46 operated in accordance with an electrical signal obtained through line 43 which connects with thermocouples 56, 52 in the bustle pipe 54. The advantage of injecting the fuel into chamber 34 rather than through tuyeres 28 is that of preventing stratitication of the gas flows, i.e., as can be seen in FIGURES 3, 5 the flow of hot air blast as fuel is in two distinct flows and it is better to have a thorough intimate mixing which is obtained by burning all the fuel in chamber 34, mixing the gases together and then passing the heated mixture of gases through the tuyeres, the flow being a homogeneous or non-stratified flow.

Generally, the two or more thermocouples are used to obtain an average temperature effective for operating the solenoid 46 and thus fuel valve 44. The bustle pipe 54 is of course connected to the hot blast main 12 in the usual manner and the circumferentially disposed tuyeres (typically eight in number connecting with the bustle pipe) insure a substantially even distribution of heated air which is adjusted compositionally to obtain the product of combustion by virtue of its in-series connection with combustion chamber 34.

It has been found that the tuyeres are incapable of sustaining substantial temperatures and to protect them against erosion from combustion of the fuel from line 30 when it is injected into contact with the flow 61 (FIG.

3) and damaging the tuyeres, there is provided a jacket 63 containing water coolant 64 which is circulated from inlet 66 into the jacket 63 and then discharged through outlet 63 so that the fuel flow indicated by reference numeral 70 will not damage the tuyeres when it discharges from passage 72 and comes into contact with the hot air blast from the bustle pipe 54.

When the stream of gases designated by reference numeral 61, and comprised of air, carbon dioxide and water, are discharged into the zone 32 they quickly fan out and produce a uniform heated condition, preventing adherence of unreduced ore on the sides of the furnace which could otherwise interfere with the analysis of the product. Referring to FIG. 5, the tuyeres flow 61 from the hot air blast is distributed through the cross section of the zone 32 to preclude such adherence of ore to the interior side walls of the blast furnace 10. The how 70, on the other hand, tends to feather upwardly (FIG. 3) and passes radially inwardly, convering at the center of the blast furnace and then moves vertically upwardly, and is reduced by the previously described endothermic process, abstracting a part of the heat of the hot air blast and adjusting its temperature to a suitable lower value which in elfect establishes an equivalent straight line temperature. By thus using several chemical conditions, it is possible to control the temperature within the zone of direct reduction rather than by putting dilution air with the flow 61 from the hot air blast and merely losing a part of the heat involved in the blast furnace operation conditions.

Concurrently, the gas which is formed, such as carbon monoxide, hydrogen and the like is suitable for reaction higher in the blast furnace at the zone designated by reference numeral 78 and commonly referred to as the indirect-reduction zone which is better adapted for reaction at lower temperatures and with gases which are in the form of hydrogen, carbon monoxide and the like. The extent of formation of carbon monoxide and hydro gen is in related proportion with the quantity of fuel obtained through line 72, which in turn is obtained from line 38. The fuel produces an endothermic effect at the direct-reduction zone 32 and is related to the temperature of the hot air blast such that the higher the temperature of the hot air blast the greater the quantity of fuel to increase the endothermic effect and thereby produces an equivalent straight line temperature to the hot blast inflow. The equilibrium between 32 and 78 is effected because at the high temperature of air blast, at which less reduction of gases is developed more fuel from line 38 is provided to maintain a more constant reducing gas concentration such that equilibrium is established even though the hot blast varies. Consequently, the two zones are at all times in equilibrium operation with each other and the burden designated as a whole by reference numeral 80 (FIG. 4) is prevented from hanging, or slipping. Because the two reaction zones are adjusted to each other the burden will pass at the proper rate vertically downwardly in the blast furnace and is successively reduced from iron ore in its highest valance form to elemental iron, passing through first the indirect-reduction zone and then the direct-reduction zone.

In operation, assuming that a straight line temperature of about 1400 F. is the straight line temperature, and assuming that the temperature in the hot blast main 12 is about 1800 F. as it is obtained from the furnace 18, the quantity of fuel obtained from line 38 and separable into branch lines 3% and 36 is adjusted so that the quantity of fuel burned in chamber 34 will generate water and carbon dioxide in a sufficient quantity to provide reducible gas in the direct-reduction Zone 32. This reducible gas, together with the flow of fuel from line 3% and incoming through tuyeres 28 and passage 72, will control the thermal and chemical reactions within the furnace. The chemical reaction in zone 32 is endothermic and adjusts the temperature of the hot blast air to an equivalent straight line value. The fuel injection, while not increasing the rate of the blast furnace, does make it possible to better regulate the equilibrium between zone 78 and 32 and, also, assists in regulating the equilibrium of the straight line air blast without diluting or wasting any of the heat derived from furnace 18. The regulation of straight line air blast is thereby accomplished without diluting or wasting any of the heatderived from the furnace 18 by means of a bypass line which was previously the case in the conventional practice. The consistently high temperature value of 1400 F. for the hot blast air insures a reduced coke rate and a high rate of iron production. Since the single largest conversion cost in the manufacture of iron is the consumption of coke, the attainment of a consistent high value hot blast air is conducive to economical operation of the blast furnace.

The total theoretical saving in coke consumption resulting from injecting various fuels is contained in the following table:

It should be noted, that the invention is not related to the production of water vapor as a parameter of control in.

the regulation of blast furnace conditions. While such control schemes have been proposed, as for example, by US. Patent No. 2,970,901 Process for Heating and Humidifying Blast for Metallurgical Furnaces, issued February 7, 1961, I have found that the use of steam as a control parameter to regulate blast furnace conditions, and derived from combustion of hydrogen-containing fuels, in an effort to maintain a predetermined constancy of humidity in the form of steam to the cold blast air, fails to embody the endothermic thermal reaction of injection' of fuel as a means for providing an equivalent straight line temperature by varying the production of reducing gases, -i.e., to obtain suitable equilibrium relation between the direct and indirect reaction zones respectively.

It should be clear from the foregoing explanation that injectants in the form of fuel or water or both can be used in combination with the hot blast line to effect a control of temperature to an equivalent straight line value. The straight line value equivalent is established as the temperature at which the blast furnace is operated at maximum or optimum efficiency. At these conditions there is a proper equilibrium relation between the distinct zones within the blast furnace. Equilibrium between the two zones insures a proper condition of the burden which avoids hanging and slippage; consequently, the blast furnace operation proceeds with greatest efficiency of use of the coke, greater production is maintained at a suitable level, and the service life of the blast furnace is held to a maximum.

To obtain these foregoing conditions of operation there is provided not only a proper quantity of injectants which is obtained responsively to temperature, but the ratio of injectants or reductants is regulated as a function of temperature to provide at the hearth zone, a chemicalthermal reaction which provides appropriate blast furnnace operation. The thermo-chemical principle which is used in con'tradistinction to the dilution of air, ortaddition of. steam, accomplishes a straight line temperature condition while wasting none of the useful heat from the hot blast line, this being in direct contrast to the previous' practices of using steam and using a cold air bypass line. It has been found that where the hot blast is raised to its straight line temperature by burning fuel in the hot air blast, such temperature is lost almost immediately by endothermic reaction brought about by reduction of the products of combustion within the furnace. Therefore, the principal value of adding injectants to the hot air blast is not to directly control the straight line temperature value,- but rather to generate a preferred quantity of product materials suitable for blast furnace operation in the zone of direct-reduction.

Although the present invention has been illustrated and described in connection with selected example embodiments it will be understood that these are illustrative of the invention and are in no sense restrictive thereof. It is reasonably to be presumed that those skilled in the art can make many revisions to suit individual design requirements, and it is intended that such revisions and modfications as incorporate the herein disclosed principles, will be included within the scope of the following claims as equivalents ofthe invention.

What is claimed is:

1. A process for producing hot air blast suitable for use in metallurgical furnaces having a direct-reduction zone and an indirect-reduction zone, comprising the steps of (a) passing a flow of air through one of a plurality of in-parallel stoves which effect heating of the air flow above a straight line reference temperature,

(b) combining with the flow of heated airta flow of reductant material which abstracts heat by endothermic reaction at the direct-reduction portion of the furnace, and V (c) proportioning the flow of reductants in accordance with temperature of the air flow to provide an equivalent to straight-line temperature value for the hot air blast and thereby provide an equilibrium of operation between the direct and indirect zones respectively of the furnace.

2. The process in accordance with claim 1 including the step of proportioning the flow of reductant in amounts whereby the greater the temperature of the air blast the greater the rate of reductant flow to absorb heat in the zone of direct reduction and thereby provide a straight line temperature effect of the, hot air blast.

3. A process for providing a blast of air which is adjusted both compositionally and in. temperature to effect balance between the direct and indirect-reduction zones of a metallurgical furnace, comprising the steps of:

(a) passing a flow of air through one of a bank of stoves which is heated sufficiently to import a heating action on the flow of air bringing it to not less than a straight line value,

(b) burning in direct combination with the heated how of air a quantity of fuel which provides as a gaseous production productv a gaseous reductant which changes the air compositionally,

(c) injecting through tuyeres a reductant which is combined with the aforesaid heated air at the direc reduction zone of the furnace to effect an endothermic reaction which provides a flow of oxidizable gas adapted for balancing the zones of direct reduction and indirect-reduction respectively.

4. A process for providing a blast of air which is adj-usted both compositionally and in temperature to effect balance between the direct and indirect-reduction zones of a metallurgical furnace, comprising the steps of:

(a) passing a flow of air through one of a bank of stoves which is heated sufficiently to impart a heating action on the flow of air bringing it to not less than a predetermined temperature,

(b) burning in direct combination with the heated flow of air a quantity of fuel which provides as a gaseous production product a gaseous reductant which controls the air compositionally,

(c) injecting through tuyeres a reductant for combining with the aforesaid heated air at the directreduction zone. of the furnace to effect an endo thermic reaction which provides a flow of reductant gas adapted for reaction within the indirect-reduction zone of said fuurna'ce, and

(d) proportioning the flow of reductants to produce conditions of temperature and composition at the direct-reduction zone of the blast furnace which obtain equilibrium between the internal zones of operation within the blast furnace comprised of the direct and indirect-reduction zones respectively.

5. A process for providing a blast of air which is adjusted both compositionally and in temperature to produce balance between the direct and indirect-reduction zones of a metallurgical furnace, comprising the steps of:

(a) passing a flow of air through one of a bank of stoves which is heated sufiiciently to impart a heating action on the flow of air bringing it to not less than a predetermined temperature,

(b) burning in direct combination with the heated flow of air a quantity of fuel which provides a gaseous reductant which controls the air compositionally,

(c) injecting through tuyeres a reductant for combining with the heated and compositionally controlled air at the direct-reduction zone of the furnace to produce an endothermic reaction adapted for reaction within the indirect-reduction zone of said furnace,

(d) proportioning the flow of reductants to provide conditions of temperature and composition at the direct-reduction zone of the blast furnace to produce equilibrium between the internal zones of operation within the blast furnace comprised of the direct and indirect-reduction zones respectively,

(e) and further proportioning the total quantity of fuel flow in accordance with the temperature of said hot air blast.

6. A process for metallurgical furnace operation, comprising the steps of:

(a) supplying a quantity of heated air which is derived from one of a series of stoves which develop within the air not less than a straight line tempera ture value,

(b) combining with said air a flow of reductant to produce a straight line temperature equivalent value of the air which produces endothermic reaction at the direct-reduction zone and thereby controls the temperature of the heated air,

(c) combining with said heated air flow prior to its injection into the furnace at the direct reduction zone a quantity of combustible fuel which is oxidized to control the composition and temperature of the air, and

(d) thereafter mixing the heated gas flow with additional reductant within the furnace to obtain metallurgical reduction in the direct-reduction zone and equilibrium operation between the indirect-reduction zone an direct-reduction zone.

7. A process for metallurgical furnace reduction operations, comprising the steps of:

(a) supplying a fiow of heated air from one of a series of heated stoves,

(b) injecting a reductant into the flow of heated air which is used in conjunction with the air to produce a straight line temperature condition effect suitable for operation at the direct-reduction zone of the metallurgical furnace,

(c) proportioning said flow of reductant in proportion to the temperature of the heated air of fuel to provide endothermic reaction which controls the term- 1h perature at the direct-reduction zone to provide straight line temperature effect for the heated air and thereby effect equilibrium between the directreduction zone and indirect-reduction zone of the furnace.

8. A process for metallurgical furnace operation, comprising the steps of:

(a) providing a flow of combustible fuel which is subdivided into two branch flows, Y

(b) burning one of said branch flows with a supply of preheated air to provide a preheated gas flow having a composition suitable for reaction at the direct-reduction zone of the metallurgical furnace, and

(c) combining the other branch flow of fuel with said gas how at the direct-reduction zone of the metallurgical furnace to generate endothermic reaction conditions which adjust the temperature conditions at the direct-reduction zone to an equivalent straight line temperature.

9. A process for metallurgical furnace operation, comprising the steps of:

(a) providing a flow of combustible fuel which is subdivided into two branch flows,

(b) burning one of said branch flows with a supply of preheated air to provide a preheated gas flow having a composition suitable for reaction at the direct-reduction zone of the metallurgical furnace,

(c) combining the other branch flow of fuel with said gas flow at the direct-reduction zone of the metallurgical furnace to generate endothermic reaction conditions which adjust the temperature conditions at the direct-reduction zone to an equivalent straight line temperature, and

(d) controlling the flow of said fuel-s together with the temperature of the preheated air reduction zone of said furnace.

10. A process for metallurgical furnace operation, comprising the steps of:

(a) providing a flow of combustible fuel,

(b) burning with said combustible fuel a supply of preheated air to provide a preheated gas flow having a composition suitable for reaction at the direct-reduction zone of the metallurgical furnace, and

(c) injecting said flows into the direct-reduction zone of the metallurgical furnace to generate endothermic reaction conditions which adjust the temperature conditions at the direct-reduction zone to an equivalent straight line temperature.

References Cited by the Examiner UNITED STATES PATENTS 1,742,750 1/ 30 Bradley -41 2,45 8,947 1/49 King et al. 2,719,083 9/55 Pomykala 7542 2,77 8,018 1/ 5 7 Strassburger. 2,970,901 2/61 Rice 754l F ORElGN PATENTS 707,391 4/ 54 Great Britain.

DAVID L. RECK, Primary Examiner.

WINSTON A. DOUGLAS, Examiner. 

1. A PROCESS FOR PRODUCING HOT AIR BLAST SUITABLE FOR USE IN METALLURGICAL FURNACES HAVING A DIRECT-REDUCTION ZONE AND AN INDIRECT-REDUCTION ZONE, COMPRISING THE STEPS OF: (A) PASSING A FLOW OF AIR THROUGH ONE OF A PLURALITY OF IN-PARALLEL STOVES WHICH EFFECT HEATING OF THE AIR FLOW ABOVE A STRAIGHT LINE REFERENCE TEMPERATURE, 